1. Field of the Invention
The present invention relates to a permanent magnet embedded motor in which a rotor provided with a plurality of slits into which permanent magnets are respectively embedded is disposed via a gap to face a stator having an iron core provided with a plurality of slots to which coils are wound.
2. Description of Related Art
A permanent magnet embedded motor is often used in an application required to rotate a rotor at a high speed. In a motor having such a constitution in which permanent magnets are embedded into a rotator, at least a pair of permanent magnets is disposed at symmetrical positions. The constitution generally used is that two pairs of permanent magnets are embedded such that their magnetic poles are positioned so as to be alternately different from each other in a peripheral direction.
In the motor having a constitution in which two pairs of permanent magnets are embedded, when the distance between the adjacent permanent magnets embedded is small, the magnetic disturbance occurs in the flow of magnetic flux inside the rotor due to the short circuit of the magnetic flux between the adjacent magnetic poles or the like. Therefore, the magnetic force generated by the mutual action with the stator attenuates and the enough torque which is required to rotate the motor is not obtained. Accordingly, the cogging torque occurs and causes to decrease the rotation efficiency of the motor.
In order to solve the problems, some prior attempts have been proposed. For example, one such prior attempt is that the center of the outer diameter of a permanent magnet embedded in a rotor is eccentric to that of the rotor so that the outer diameter of the outside contour of the permanent magnet is smaller than the outer diameter passing through apexes of the adjacent permanent magnets to make the thickness of the adjoining portion of the permanent magnet thinner. Therefore, the configuration of the rotor is formed in a petal shape and thus the short circuit of the magnetic flux is reduced and a smooth rotation without torque irregularity can be attained (for example, refer to Japanese Patent Laid-Open No. 2000-350393).
Another example is that gap portions are formed on both sides of the slit of a rotor to which a permanent magnet is embedded to magnetically insulate the adjacent magnetic poles from each other and thereby the short circuit of the magnetic flux and the attenuation of the magnetic force which is the source of a torque are prevented (for example, refer to Japanese Patent Laid-Open No. Hei 05-236684 and Japanese Patent Laid-Open No. 2000-069717).
In addition, in the motor having a constitution in which a permanent magnet is embedded in a rotor as described above, a hole is formed at a position where the flux change in a rotor core is little and a pin or a bolt is provided so as to pass through the hole to integrate laminated rotor cores together firmly. Thereby, the disturbance of the magnetic flux in the rotor core is reduced and the rotation efficiency of the motor can be enhanced (for example, refer to Japanese Patent Laid-Open No. Hei 05-236686).
In the above-mentioned conventional permanent magnet-embedded motors, the cogging torque and the counter-electromotive force distortion factor are reduced to some extent. However, the configuration of the rotor near the place where the polarity of the magnetic flux distribution is inverted is required to improve in order to reduce the torque ripple, noise or vibration.
Especially, the air gap portion provided at a prescribed position in the rotor for preventing the short circuit between the adjacent permanent magnets may be effective for preventing the attenuation of the torque required for rotation and for decreasing the torque irregularity. However, the shape and the size of the air gap portion provided in the rotor are the important elements which have influence on the size of the entire motor and thus it is important to obtain its optimal configuration and size. When the air gap portion is simply made larger, a useless portion is formed in the motor. On the contrary, the gap portion is simply formed to be smaller, the effects for preventing the short circuit of the magnetic flux and for decreasing the torque irregularity may not be expected. Therefore, it is important that the configuration of the rotor or the configuration of the air gap portion provided in the rotor is formed to be optimal.
In addition, the rotor is constituted by laminating a plurality of rotor plates in the manufacturing process. The rotor plate is, for example, formed by punching out from a silicon steel plate with a press machine or the like. In order to form the configuration of the rotor in the petal shape, the rotor plate is required to be formed in a petal shape
However, the rotor plate is formed in such a manner that a plurality of rotor plates are successively punched out from a sheet of silicon steel plate. Therefore, in the case that the contour of the rotor plate is formed in a petal shape, it is difficult to perform punching without being provided with a space between the adjacent rotor plates to be formed from the sheet of silicon steel plate. For example, even when the adjacent rotor plates are punched out in the state that they are disposed without a gap space therebetween in the silicon steel plate, two rotor plates can come into contact with only one point and the other portions are separated from each other, which causes to form a useless space. Therefore, the steel plate material of the portion corresponding to the useless space is not used as the rotor plate and thus the sufficient utilization of material cannot be attained.
Furthermore, in the conventional permanent magnet-embedded motor, even when the hole is formed at the position apart from the flux change, in the case that the fixing member such as a pin or a bolt which integrates the laminated iron cores is made by magnetic substance, the iron loss (hysteresis loss and eddy current loss) may occur, which causes to lower the rotation efficiency. Besides, when the configuration of the rotor near the place where the polarity of the magnetic flux distribution is inverted is not improved and the outer peripheral configuration of the rotor is not improved, the cogging torque and the counter-electromotive force distortion factor remain larger and thus it is insufficient to reduce noise and vibration.